The horizontal stabilizer moves as one complete unit to provide pitch trim. In order to remain flush with the fuselage throughout the full range of motion, the fuselage is flattened in that area. It appears to be concave in comparison to the curved fuselage around it, but I believe it is indeed flat.

could it be improved airflow? i imagine the air towards the back of the aircraft would be more turbulent?

As mentioned, this is where the stabilizer rotates for trim control. It is mostly flat. The fuselage tapers behind this area, so there is no need for it to be flat. There is 1-2 inches of clearance between the stabilizer and fuselage skin.

Quoting faz777 (Thread starter):
noticed similarities on other aircraft but the 747 looks to have the largest.

The 747 stabilizer is the biggest and moves the most.

If you have never designed an airplane part before, let the real designers do the work!